To me the most fun part about fix-its is placing dominoes.
Tragedies often consist of escalating series of actions and circumstances which, in isolation, were not clearly leading to the tragic end but form a chain of cause-and-effect directly towards it in hindsight. In equal but opposite fashion, I love starting with small inoccuous changes to canon that in themselves do not obviously fix everything but start a new chain that leads to a better ending.
It's kind of impossible for fix-its to feel fully natural– the reader by definition knows what the original ending was and that this ending will be happier because the writer wants it to be– but it is possible for them to not feel contrived. A big deus-ex-machina, or a character breaking with their pre-established tragic flaws to suddenly make all the "correct" decisions almost always feels unsatisfying to me.
But a few carefully placed small domino pieces slowly knocking over bigger and bigger tiles until the entire story has radically changed? That's a lot more fun.
It recquires the author to both correctly identify the original chain of cause-and-effect and understand the characters well enough to know how they'd react to different circumstances. Because if the story feels like it's fixing the wrong problem or the characters don't act like themselves the magic is lost. But when it works? When it clicks and the reader sees the domino chain laid out in front of them? It's beautiful.
